In the manufacture of semiconductor devices, wafers, such as silicon wafers, are subjected to a number of processing steps. The processing steps include depositing or forming layers, patterning the layers, and removing portions of the layers to define features on the wafer. One such process used to form the layers is a procedure known as chemical vapor deposition (CVD), where reactive gases are introduced into a vessel containing the semiconductor wafers. The reactive gases facilitate a chemical reaction that causes a layer to form on the wafers.
One deposition process involves the formation of polycrystalline silicon (polysilicon) layers on the wafer by decomposing SiH4 molecules to Si atoms, which in turn combine to form Si grains, or “polysilicon.” There are numerous factors that affect the deposition rate and deposition polysilicon film characteristics of a deposition tool. These factors include, e.g., the flow rate of reactive gases through the chamber and the temperature/pressure of the chamber.
Polysilicon furnaces both within and without a nitrogen box (the furnace wafer loading area immediately below the furnace main body) may play an important role in controlling polysilicon grain size, thereby governing device speed. An important consideration is the actual N2 (nitrogen) flow which continuously treats the silicon wafer surface in the N2 box. A continual need exists for effective mechanisms for increasing the grain size of a silicon wafer.
The structure of films and the electrical behavior of devices incorporating the polysilicon layers are influenced by deposition conditions. Also, diffusion in polysilicon layers is influenced by grain boundaries. The grain boundaries are affected by e.g., the deposition temperature, pressure, and gas that individually influence the Si-seed number at the seed layer. The more seeds that exist, the smaller the growth of grain size, and the more grain boundaries are formed. By the method of the present invention, the oxygen molecules, which will be interfered by N2 flow in the N2 box, adsorb on the oxide surface before “boat up” influences the poly seed number, and thus the grain size.